Field of the Invention
The present invention relates to an encoder that produces position information by using a scale and a sensor movable relative to each other.
Description of the Related Art
In such an encoder, a periodic pattern (hereinafter, referred to as a scale pattern) provided on a scale at a predetermined pitch in a direction of relative movement between the scale and a sensor is read by an optical, magnetic or conductive sensor to produce a periodic signal. Calculation by using this periodic signal then produces position information such as a relative position of the scale and the sensor and an absolute position thereof. Thus, any cumulated error (hereinafter, referred to as cumulated pitch error) in the direction of relative movement in a period (pitch) of the periodic pattern reduces a position detection accuracy of the encoder.
Japanese Patent Laid-open No. 06-317431 discloses an encoder including multiple detection heads disposed on a sensor and configured to read a scale pattern. The detection heads are disposed with an interval therebetween in a direction of relative movement between the scale and the detection heads. The encoder uses a measured value obtained through the detection heads to calculate a cumulated pitch error of the scale pattern. The cumulated pitch error thus calculated can be used to calibrate the encoder.
However, any alignment error, which is an error in alignment of the scale and the sensor, causes a line connecting positions at which the detection heads read the scale pattern to be tilted relative to the scale. This prevents the cumulated pitch error of the scale pattern from being accurately calculated, so that the calibration using the cumulated pitch error cannot improve the position detection accuracy of the encoder.